Natural Hazards and Earth System Science
www.nat-hazards-earth-syst-sci.net
1561-8633
1684-9981
7
2
2007
10.5194/nhess-7-207-2007
http://www.nat-hazards-earth-syst-sci.net/7/207/2007/
http://www.nat-hazards-earth-syst-sci.net/7/207/2007/nhess-7-207-2007.html
http://www.nat-hazards-earth-syst-sci.net/7/207/2007/nhess-7-207-2007.pdf
207
218
2007-03-05
Debris flows on forested cones – reconstruction and comparison of frequencies in two catchments in Val Ferret, Switzerland
M. Bollschweiler
michelle.bollschweiler@unifr.ch
M. Stoffel
Laboratory of Dendrogeomorphology, Department of Geosciences, University of Fribourg, Chemin du Musée 4, 1700 Fribourg, Switzerland
Debris flows represent a major threat to infrastructure in many regions of
the Alps. Since systematic acquisition of data on debris-flow events in
Switzerland only started after the events of 1987, there is a lack of
historical knowledge on earlier debris-flow events for most torrents. It is
therefore the aim of this study to reconstruct the debris-flow activity for
the Reuse de Saleinaz and the La Fouly torrents in Val Ferret (Valais,
Switzerland). In total, 556 increment cores from 278 heavily affected
<i>Larix decidua</i> Mill., <i>Picea abies</i> (L.) Karst. and
<i>Pinus sylvestris</i> L. trees were sampled. Trees on the cone of Reuse
de Saleinaz show an average age of 123 years at sampling height, with the
oldest tree aged 325 years. Two periods of intense colonization (the
1850s–1880s and the 1930s–1950s) are observed, probably following
high-magnitude events that would have eliminated the former forest stand.
Trees on the cone of Torrent de la Fouly indicate an average age of 119
years.
<br><br>
As a whole, tree-ring analyses allowed assessment of 333 growth disturbances
belonging to 69 debris-flow events. While the frequency for the Reuse de
Saleinaz study site comprises 39 events between AD 1743 and 2003, 30 events
could be reconstructed at the Torrent de la Fouly for the period 1862–2003.
Even though the two study sites evince considerably different
characteristics in geology, debris-flow material and catchment morphology,
they apparently produce debris flows at similar recurrence intervals. We
suppose that, in the study region, the triggering and occurrence of events
is transport-limited rather than weathering-limited.
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